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You were diagnosed with an AHI of 32. Your machine says 2.4. Both are called AHI, so it is natural to treat them as the same ruler. They are not, and understanding why makes it easier to judge your own numbers.
Key takeaways
- Diagnostic AHI describes your untreated apnea; residual AHI is what is left while on therapy.
- A machine estimates events from airflow alone; a sleep study uses oxygen and brain activity too.
- Compare yourself with your own baseline and the usual under-5 target, not with the lab figure.
Two quantities with one name
Before therapy a sleep study produces a diagnostic AHI. For adults the usual bands are under 5 normal, 5 to under 15 mild, 15 to under 30 moderate, and 30 or more severe. Once you are on CPAP, the number you see each morning is a residual AHI: the events that still occurred while the machine was running. Someone who started at 60 and now runs at 4 has had an excellent response. See what is a good AHI on CPAP for the target most clinicians use.
Why the two numbers are not like for like
| Sleep study (diagnostic) | CPAP machine (residual) | |
|---|---|---|
| Sensors | Airflow, effort belts, oxygen, brain waves, heart rate | Airflow inside the mask only |
| Denominator | Hours of sleep (in-lab) or recording time (home test) | Hours the mask was on |
| Hypopnea rule | Scored by a technologist against a stated rule (3 or 4 percent oxygen drop, or an arousal) | Flow-based; the machine cannot see oxygen or arousals |
| Scored by | A trained person, by hand or with software | The machine's algorithm, in real time |
| Cannot see | Nothing for the night it covers | Events that reduce oxygen without a clear flow change |
The mask-on denominator is a quiet difference: a night with two awake hours in bed with the mask on counts those hours, which tends to lower the number slightly. Our guide to hypopnea scoring explains why the rule matters.
Reading your residual AHI well
Look at three things together. First, the level: most people aim for under 5. Second, the mix: mostly obstructive events call for different thinking than mostly central ones, and central versus obstructive apnea on CPAP covers the difference. Third, leak and usage: a low AHI on a high-leak night may mean the machine could not see well, and a low AHI over a short night is not much comfort.
Not medical advice. Apnealytics is a tracking and coaching tool, not a diagnostic device. Machine-estimated numbers are not a clinical diagnosis; talk to your clinician about your therapy.
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Written by
Haniel Singh
Founder of Apnealytics. A CPAP user who builds software, not a sleep physician.
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